This invention relates to electric motors and in particular, to an end cap for a pmdc motor.
Two speed pmdc motors are known where the speed change is obtained by using a three brush construction where one brush is common and the other two brushes are selectively used for low speed or high speed operation.
The low speed brush is generally located 180° opposite the common brush and the high speed brush is located between the other two brushes, generally about 90° from the common brush. Such an arrangement works well for a single direction motor but for a two speed bi-directional motor, there is a big difference in the no load speed between clockwise and counterclockwise high speed operation. This has been generally tolerated as a failing of a two speed pmdc bi-directional motor.
Attempts to correct this undesirable characteristic include the use of two high speed brushes, one for each direction, resulting in a four brush system. This provides a satisfactory solution but the four brushes are fixed to the end cap assembly in predetermined spatial locations. The speed of the motor in high speed mode can be adjusted slightly by changing the angular displacement or location of the high speed brushes with respect to the common brush. As end caps are molded parts with the brush cages being an integral part of the end cap, to change the speed of the high speed mode required a new end cap to be formed requiring a new end cap die, etc.